RFR: Add regular regions to free set after partial GC

[Aleksey Shipilev]

On 08/24/2017 07:32 PM, Aleksey Shipilev wrote:
> This is the leftover from LRU/Generational work. It was deemed necessary to avoid allocating in
> non-empty regions to keep allocation timestamps in order. After some discussion, it seems that we
> can still allocate in regular regions after partial GC.
> 
> Now, the tricky part is that we have to still add root regions to _root_regions, otherwise
> everything breaks. Note that this fix means root regions are in free set, which means we could
> evacuate objects from cset to root regions. I think this is not problematic -- it fact, preferring
> root regions over other reduces heap connectedness?

Complication: we need to bound the root region scan at its latest top:
  http://cr.openjdk.java.net/~shade/shenandoah/partial-root-free/webrev.01/

-Aleksey